This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Blowout preventers are used extensively throughout the oil and gas industry. Typical blowout preventers are used as a large specialized valve or similar mechanical device that seal, control, and monitor oil and gas wells. The two categories of blowout preventers that are most prevalent are ram blowout preventers and annular blowout preventers. Blowout preventer stacks frequently utilize both types, typically with at least one annular blowout preventer stacked above several ram blowout preventers. The ram units in ram blowout preventers allow for both the shearing of the drill pipe and the sealing of the blowout preventer. Typically, a blowout preventer stack may be secured to a wellhead and may provide a safe means for sealing the well in the event of a system failure.
A typical blowout preventer includes a main body with a vertical bore. Ram bonnet assemblies may be bolted to opposing sides of the main body using a number of high tensile bolts or studs. These bolts are required to hold the bonnet in position to enable the sealing arrangements to work effectively. Typically an elastomeric sealing element is used between the ram bonnet and the main body. There are several configurations, but essentially they are all directed to preventing a leakage bypass between the mating faces of the ram bonnet and the main body. Each bonnet assembly includes a piston which is laterally movable within a ram cavity of the bonnet assembly by pressurized hydraulic fluid acting on one side of the piston. The opposite side of each piston has a connecting rod attached thereto which in turn has a shear ram and corresponding blades mounted thereon.
These rams are designed to move laterally toward the vertical bore of the blowout preventer to shear or cut any object located therein. For instance, the rams can close in on and shear a tubular within the vertical bore of the blowout preventer, such as a section of drill pipe used during drilling operations. The opposing rams typically experience some axial separation after shearing, particularly when shearing a larger object such as a tool joint. The axial separation results from shear forces encountered when shearing the larger object, leaving a vertical gap between the opposing shear blades.
After shearing, the rams can be withdrawn from the bore to allow for operations to once again be conducted through the blowout preventer bore. As an example, tools may be lowered through the blowout preventer bore on wireline. In some instances, the rams may be needed again to shear or cut the wireline located in the blowout preventer bore. This can be difficult, especially if wireline needs to be cut by rams that experienced axial separation during shearing of a tubular, because wireline tends to stretch rather than cleanly shear when the rams are closed.
Accordingly, a mechanism for enabling shear rams to more efficiently shear an object located in a blowout preventer bore, such as wireline, after previously shearing one or more other objects is desirable.